Load a compilation unit from disk based on a classname. A class file is parsed if one exists
matching the classname that is not older than a corresponding source file, otherwise the
source file is selected.
This method is called by the LibCompilationUnit NTA. We rely on the result of this method
being cached because it will return a newly parsed compilation unit each time it is called.
- the loaded compilation unit, or the provided default compilation unit if no matching
compilation unit was found.